18 U.S. Code § 1731 - Vehicles falsely labeled as carriers

It shall be unlawful to paint, print, or in any manner to place upon or attach to any steamboat or other vessel, or any car, stagecoach, vehicle, or other conveyance, not actually used in carrying the mail, the words “United States Mail”, or any words, letters, or characters of like import; or to give notice, by publishing in any newspaper or otherwise, that any steamboat or other vessel, or any car, stagecoach, vehicle, or other conveyance, is used in carrying the mail, when the same is not actually so used.

Whoever violates, and every owner, receiver, lessee, or managing operator who suffers, or permits the violation of, any provision of this section, shall be fined under this title or imprisoned not more than six months, or both.